In vivo determination of the centres of resistance of maxillary anterior teeth subjected to retraction forces

This study was designed to locate the centres of resistance of consolidated units of two, four, and six anterior teeth during retraction in two human subjects. Initial displacements of these units were separately measured when retraction forces were applied at different levels by means of a device for displacement measurement using magnetic sensors and magnets. By calculating the angle of rotation from the displacements measured, the location of the centre of resistance was determined for each unit. The results showed that the centres of resistance of the two- and four-incisor units were approximately at the same position, whilst that of the six-tooth unit was observed to be more incisal. Clinically, this finding indicates that translation can be achieved with a smaller amount of moment-to-force ratio in en masse retraction than in two- or four-incisor retraction. The results also indicate that the location of the centre of resistance of the anterior segment during retraction may depend on the palatal alveolar bone height, rather than on the labial alveolar bone height.